Dietary and developmental regulation of intestinal sugar transport.

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.